Kepler Finds Earth-Sized Planets

by Paul Gilster on December 21, 2011

I’m delighted that we keep finding solar systems so different from our own. The discovery of two new planets that are roughly the size of the Earth just confirms the feeling — in a galaxy of dazzling fecundity, every system we look at has its own peculiarities to instruct and delight us. The system around the star called Kepler-20 (from its designation by the space observatory studying planetary transits) is a case in point. Yes, it has small, rocky worlds, but it also has three larger planets, and all five orbit closer than the orbit of Mercury in our own system. Kepler-20 is a G-class star somewhat cooler than the Sun located some 950 light years from Earth in the constellation Lyra.

Moreover, while we once assumed that smaller planets orbited close to stars while larger gas giants orbited further out in the system (again based on our own system and our assumptions about it), our new discoveries point to different scenarios. In Kepler-20 we have a system where the larger planets (all smaller than Neptune) orbit in alternating fashion with the rocky planets. We get a big planet and then a little one, then another large one, another rocky planet, followed by a third large world. Kepler 20-b, 20c and 20d are the large planets, with diameters of 24,100, 39,600, and 35,400 kilometers respectively, and orbital periods of 3.7, 10.9, and 77.6 days.

“The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system,” said Jack Lissauer, planetary scientist and Kepler science team member at NASA’s Ames Research Center in Moffett Field, Calif. “The analysis of Kepler data continues to reveal new insights about the diversity of planets and planetary systems within our galaxy.”

As you would expect, it’s the small planets that command the headlines this morning, their presence seen as another step in the goal of finding an Earth-sized planet in the habitable zone. These worlds at least fit the bill in terms of size, although water on the surface is out of the question, as temperatures are thought to reach 760 degrees Celsius on the inner world and 426 degrees Celsius on the outer. With an orbit of 6.1 days, Kepler 20e is equivalent to 0.87 times the size of Earth, with a diameter of 11,100 kilometers. Kepler-20f, orbiting the host star every 19.6 days, has a diameter of 13,200 kilometers, quite close to the size of the Earth. Both planets are expected to be rocky, with masses less than 1.7 and 3 times that of Earth.

Image: This chart compares the first Earth-size planets found around a sun-like star to planets in our own solar system, Earth and Venus. NASA’s Kepler mission discovered the new found planets, called Kepler-20e and Kepler-20f. Kepler-20e is slightly smaller than Venus with a radius .87 times that of Earth. Kepler-20f is a bit larger than Earth at 1.03 times the radius of Earth. Venus is very similar in size to Earth, with a radius of .95 times that our planet. Prior to this discovery, the smallest known planet orbiting a sun-like star was Kepler-10b with a radius of 1.42 that of Earth, which translates to 2.9 times the volume. Credit: NASA/Ames/JPL-Caltech.

So there we are, the smallest planets yet confirmed around a Sun-like star, a likely case of planetary migration in which planets form much farther from the host star and migrate inward because of interactions with the protoplanetary disk. Although Kepler’s transit methodology is finely tuned, the radial velocity signature of planets like the smaller two around Kepler-20 is out of the detection range of current technology. Kepler’s ‘long stare’ at the starry patch in Cygnus, Lyra and Draco was able to turn up the planetary candidates here, but intense computer simulations by way of follow-up were necessary to demonstrate the likelihood of the smaller detections being planets.

The universe and particularly planetary systems are even more surprising and intriguing than we could have suspected.

But, Paul, with all due respect, I think I do not entirely agree with your description of alternatingly a smaller and larger planet; as you know the designations b, c, d, e, f are not necessarily from inside outward, but in order of discovery.

Ronald, I was pulling this from a CfA news release, specifically this:

“They also show an unexpected arrangement. In our solar system small, rocky worlds orbit close to the Sun and large, gas giant worlds orbit farther out. In contrast, the planets of Kepler-20 are organized in alternating size: big, little, big, little, big.

“We were surprised to find this system of flip-flopping planets,” said co-author David Charbonneau of the CfA. “It’s very different than our solar system.”

We do have two Earth-like worlds here (in terms of size), rather than just one, as in your comment above.

Another amazing exoplanet discovery! Clearly, our knowledge of other planetary systems has exploded since the advent of Kepler. And a pattern seems to be emerging from its discoveries as well as from the high precision radial velocity planet searches: systems with close in planets are very common. This means that migration is occurring frequently or there are stars with enough material around them that close in to form these planets roughly where we find them eons later. Probably both of these formation mechanisms are at work. I will be curious to see whether the habitable zones of the Kepler stars are as richly populated as the inner areas, or, might there exist a drop off in planet counts near the HZ, as a HZ planet desert could help solve the Fermi paradox?

Lastly, given the progress being made by Kepler, it would be sheer folly for this mission not to be funded for another few years.

This star system piqued my interest more than the recent disclosure of Kepler-22 b — the so-called “Earth-sized” world that’s actually 2.4x bigger than the Earth. Now we can indeed say that Kepler can detect truly Earth-sized worlds–a notable milestone.

However, with a period of just 6 & 20 days respectively, we may infer that it took a LOT of transits for Kepler-20 e & f to be sniffed out of the data. To find a planet the size of Earth or Kepler-20 f at a “habitable” orbital distance of Earth or Kepler 22 f may indeed require a years-long extension of the Kepler mission as previously noted on Centauri Dreams.

Is there a current view on how early in the history of the universe it is likely that metallicity was sufficiently high to allow formation of terrestrial type ( i.e. rocky) planets? Clearly by at least 8.8bya, but I ask as I am clearly out of date and continue to be surprised by how old some of these planetary systems are…

The Kepler findings continue to be really exciting – in an abstract way because of the great distance of these objects.

@ Paul Gilster – Sir, is there any possibility of a brief summary or list of teams presently known to be working to identify planetary systems around stars in the vicinity of Earth, say <25 light-years, and their data-gathering methods?

I cannot help elaborating a bit further on the innermost two planets (whichever they are): their orbits (or rather their semi-major axes) are only 0,006 AU apart, that is hardly a million km, or less than 3x earth-moon distance! And virtually the same inclination (as may be expected).

That is what I call close. How could these orbits ever have been so stable for gigayears?

paul, I think I found the reason for the discrepancy in your post (alternating bigger-small-bigger-small-bigger) and mu comment, see the publication “Two Earth-sized planets orbiting Kepler-20″ by Fressin et al. of today;
The discrepancy is entirely in planet f (number 4 from inside out):
according to the authors its mass is larger, < 14.3 Me using 'spectroscopic limit', but only 0.66 Me up to 3.04 Me, using 'theoretical considerations'. The authors choose the latter, whereas the Extrasolar Planets Enc. used the first, higher, mass estimate.
If we take the authors' lower estimate, the mass order is indeed alternating as your post mentions.

I wonder if these inner worlds are tidally locked, then the darksides should be quite cool – maybe they could have water/ice on their darksides if the atmosphere is thin enough to transport some of the heat from the sunward side. Just a thought

@ Paul Gilster – Sir, is there any possibility of a brief summary or list of teams presently known to be working to identify planetary systems around stars in the vicinity of Earth, say <25 light-years, and their data-gathering methods?

This would make a good article here, and I’ll look into it. What first comes to mind are the three teams looking hard at Alpha Centauri, about which we may have results in the coming year. But widening this out to 25 light years is an interesting exercise. Maybe the readers can chime in on this, and in the meantime I’ll look into putting something together for January that talks about who’s looking at what relatively close to home.

Kepler-20s’ remarkable planetary features and spacings are certainly going to give the planet formation theorists something to work on. Oh well, astronomers are supposed to have sleepless nights. The tight spacing of two of those orbits (if they really have the data pinned down and I think they do) is truly amazing.
Have these orbits really been stable over the estimated life of this system?
Kepler just keeps surprising us!

The system looks very unlike our own, but why they say a new mechanism is needed to explain it’s formation? Possibly some density waves in the protoplanetary disk?.. I recall something resembling this was proposed for the formation of Saturn’s satellites, with their densities alternating between water-like ~1 g/cm3 for Mimas, Tethys and Rhea, and more rocky ~1.5 g/cm3 for Enceladus, Dione and Titan.

And besides, the system appears to be very similar to Kepler 11 where all 5 inner planets are coplanar and very tightly packed, with two inner planets especially close to each other! And the alternating pattern, which is less pronounced, but it is still there, 4 Me, 13 Me, 6 Me, 8 Me and 2Me…

To Erik Anderson, the automated planet finder (APF) in California will be looking at the brighter and closer stars using the radial velocity method. They have some very advanced adaptive optics and spectrographic equipment.
I believe they will start observing early in 2012 on a target list that includes all nearby stars visible from that latitude. Though it’s a ground based scope one hopes that using a dedicated telescope with many repeated observations that Earth radius sized exoplanets might be detected. It’s a poor mans TPF.

“we may infer that it took a LOT of transits for Kepler-20 e & f to be sniffed out of the data. ” – Erik Anderson

Indeed. While Kepler is certainly a resounding success in producing statistics in the Jovian-Neptune-SuperEarth radius classes, the holy grail of getting an accurate census of Earth radius planets with this instrument seems to be most unlikely.

The legacy of Kepler will likely not be any specific planet discoveries, but the demonstration that planetary systems of all types are present, and the funding of next generation planet finders is justified.

@Interstellar Bill – Not sure there is any typical type of planetary system, but still no reason to think that ours is unusual. Concerning Kepler, even if it was observing our system along the ecliptic plane, there would be zero detections. Mercury is too small. Venus and Earth have too long periods to have enough transits in mission time to be picked out of the noise.

I wonder if too much is being made of the supposed ‘anomalous’ distribution of planet sizes here. Per the reports, of the three gas ‘giant’ planets here two are substantially smaller than Neptune, and just one approaches the size of Neptune, which gives us a system of basically two earth-terrestrials, two mini-neptunes, the smaller of which might even be a super-earth’, and one hot Neptune, all very close in to the star.

This system may not actually violate the expected small-in-close/large-further-out pattern. Instead of big-small-big-small-big, it might actually be small-small-small-small-small, with perhaps as yet undetected larger planets further out (or not). After all, one expects some variability in sizes even if there is conformation to the general pattern, and even in our own solar system, Mars is further out than Earth, but smaller. (And I recall quite a few other multi-planet systems are known to have at least one biggish planet closer in than further out smaller planets)

Also, if the observed orbital positions are the result of substantial amounts of inward migrations, one would kind of expect any pre-existing size relationships based on formation distance to have been thoroughly shredded during the migration phase anyways.

Ronald: their orbits (or rather their semi-major axes) are only 0,006 AU apart, that is hardly a million km, or less than 3x earth-moon distance! And virtually the same inclination (as may be expected).

The tides must be enormous when the inner planet passes the outer.

Michael: I wonder if these inner worlds are tidally locked, then the darksides should be quite cool – maybe they could have water/ice on their darksides if the atmosphere is thin enough to transport some of the heat from the sunward side

Tide-locked planets with small habitable regions are entirely possible, as are habitable sub-ice oceans much farther out in solar systems. The “habitable zone” is an extremely incomplete characterization of habitability.

Ronald, there’s something seriously wrong with the exoplanet.eu data at present: if you look at the transit periods (which are correct), it’s obvious that the semi-major axes don’t follow Kepler’s third law. They also had the radius of e wrong yesterday, though that’s now corrected.

The papers are on arxiv now (1112.4514 and 1112.4550), but it’s tricky to reconstruct a view of the system as a whole, because b, c, d are in one paper (ApJ) and e, f in the other (Nature); the Nature paper only gives semi-major axes in terms of the stellar radius.

There’s something about the dubious effect of Nature on the scientific communication process in there, but I won’t try to draw it out.

“Interstellar Bill :
” Before exoplanet discoveries the confident assumption was Sagan’s Copernican Principle, of the typicality of our solar system.
I’m waiting for the Consensus to admit that CP is totally out the window.”

It’s just the other way around. I am sure that what Sagan meant was just the opposite about the CP.
Supposing that we would find planetary systems such as ours common then that would be a violation of the CP.
We have always been wrong when we took the view that we were unique, be that biologically or cosmologically.
Sagan would be delighted with the situation now. Matters not the configuration of the planets, it is that planetary systems are common.
The statistics are closing in on there being 10’s of billions of planets in the Galaxy. (That even makes the odds better that some fraction of them are solar system like.)

Ignoring the CP has probably led planetary system formation astrophysics people astray.
Starting with a protoplanetary disk there must be an unknown number of ways to get to a planetary system. Subconsciously constraining planet formation to produce ‘something’ like the solar system is a violation of the CP.
The CP rubs our nose in our non-uniqueness all the time.

Interesting to note, planet formation and orbital evolution from disk to planets is a chaotic process. That could mean that 800 billion number of ‘rogue Jupiter’s’ could be way underestimated, maybe a lot of planets from the size of Mercury to Jupiter wander the lanes in the Galactic arms.

@Daniel: yes, those were exactly the two publications I used and referred to.
BTW, the EP Encyclopedia did not really mess up the planet mass data, the quoted data are within the ranges mentioned in the two pubs.

And as I mentioned, the only deviating one, planet f can easily be explained by the EPE using the ‘spectroscopic limit’, and the authors using a much smaller mass based on additional ‘theoretical considerations’. However, both are mentioned in the publication.

It’s truly exciting that so much data is coming in about exoplanets. Although these terrestrial planets are too hot, this is promising for the abundance of terrestrial planets in the galaxy. It’s only a matter of time before we find earthlike planets ranging from steamy jungles to snowballs.

It’s important to keep in mind that an earthlike planet in the HZ is a great challenge to find – our methods are going to favor planets that are large, close orbiting, or especially both. It follows that we’d find all those hot jupiters, and that the smallest planets yet are found close-in.

I feel very optimistic about the exoplanet “holy grail” of other earths, as long as the funding and effort for these fruitful searches keeps coming in.

All stars are solar type (G) of 0.9 – 1 solar mass and all but one (Kepler-10) have solar to higher metallicity (0 – 0.2). All stars are at least 3 gy old to much older.
They all have superearth to Uranus/Neptune sized planets with orbital separations from (rounded off) 0.01 – 0.24 AU.

Besides the already mentioned innermost two planets od Kepler 11 and 20, it is also remarkable that Kepler 18 and 20 have Uranus sized planets with separations of only resp. 0.04 and 0.02 AU.

And maybe even more amazing, Kepler-9 has two Saturn sized planets with only 0.09 AU separation!

Apparently planetary systems can be quite crowded and yet have long-term orbital stability.
It seems unlikely that this stability could persist with such crowded close-in systems, if they were the result of inward migration, as amphiox also points out.
And, as nick points out, mutual tidal effects must be significant anyway, during passing. I estimated that the total gravitational pull of the two ‘Saturns’ of Kepler-9 at closest approach is about 150 times that of the earth-moon combination (M1*M2/R^2).

Further to my previous, with regard to tidal effects: the gravitational pull of the innermost two planets (b, e) of Kepler-20 and that of the two Uranus-sized planets c and f, at closest approach, is over 200 times the earth-moon gravitational pull.

I am more hopeful about the starshade option for JWST finding planets within 10pc than radial velocity and astrometry programs because low-mass planets may show up easily in a JWST image. The other methods have higher thresholds of detection in terms of planetary mass (hence, they will leave Earth-sized planets undetected).

I guess there could be enough sunlight reflected off the outer planet in such tight “pairs” at least to boil off some frosen atmospheres at the nightside of inner planet. If the latter is, for example, a (super)earth with thin envelope of phodissociated O2 at 0.04 AU from sunlike star, and the outer is a neptune with 20000 km diameter at 0.045 AU, then the latter would recieve ~1 MW/m2 of insolation, and reflect around 300 kW/m2 (assuming it is not very dark like TrES-2b), with total power of reflected sunlight around 4*10^20 W. At opposition, it would mean ~100 W/m2 on the nightside of inner planet, which is greater than the insolation of Jupiter and corresponds to equilibrium temperature around 150K, depending on albedo. But it is much higher than boiling point of oxygen and can even sublime come CO2. The outer planet would shine at -24 mag in the sky of inner planet at opposition! and I start to think that our Earth skies are very boring by galactic standards…

PS of course there would be only brief periods of illuminations at the oppositions, but even if the average illumination is 10 times lower, and no internal heat is present (which is very unlikely!), oxygen/nitrogen would not condense.

As also mentioned in the recent thread about the ‘Planetary Jungle’, it makes one wonder why there are so many close-in super-earths and ‘subgiants’ (are they really ice giants that close in?).
Are they the rocky remnants of gas and ice giants whose gaseous/liquid envelopes have been blown away by the host star, or did those envelopes not form at all, being well within any snow-line or the like?
And why so many large rocky planets in the inner system at all (I mean with or without lighter envelopes)?
There seems to be little correllation with stellar mass (at least for solar type stars above a certain threshold mass), but there could be a stronger one with stellar metallicity.

i do not even get why we spent so much time on finding planets around stars that are more than 500 lightyears away. Alpha centauri is the best system to find life. Why not spent more money to find planets there. It is a very high chance to find a earth size planet with life there

Is there any possibility that the interpretation of the data for planets with orbital radii with little separation is incorrect, and that what we are really looking at is binary planets with the same orbit?

I am assuming the data shows little dips that can be interpreted as 2 sets with different repeat frequencies and differing phases between the 2 series indicating different orbits. Is that the case, or is there any indication that the data shows coupling of the periodicities?

Joy
” The legacy of Kepler will likely not be any specific planet discoveries, but the demonstration that planetary systems of all types are present, and the funding of next generation planet finders is justified.”
If Kepler only funktions in its original minimal design lifetime , that wil be true . The big question is how long it could funktion if ” evrything goes right “.
Engineers are often very conservative in their estimate of design lifetime, it seems probable that Kepler could go on funktioning as designed for another extra 4-6 years at a cost of 20mil ayear . Perhabs it could be extended even further if reprogrammed to conserve resources as by concentrating on a limited number of highpriority tasks .

This is a great example of how once-trumpeted establishment dogmas are never given a proper funeral, one that apologizes to all the doubters, who turned out to be correct.
In this case it’s Carl Sagan’s beloved Copernican Principle, which has been decisively disproved by our own neat & tidy solar system being overwhelmingly untypical.
If, say, the expanding-universe paradigm is ever abandoned, it would be a similar matter of everybody sneaking out the back door and quietly turning the lights off, but never apologizing to Halton Arp.

@Bill: As I understand it, a system like ours would not have a single planet detected by Kepler. The large majority of stars observed by Kepler do not show evidence of planets, for various reasons. There is therefore no evidence whatsoever from Kepler that ours is not a typical system. The galaxy could be full of ones just like ours, and we would not know about it. Nor do we have any evidence to exclude or even just disfavor this possibility.

Interstellar Bill, all this is not as extraordinary as you make it sound, but it is the way science proceeds: falsifying older theories in favor of new ones in the light of new observations and insights.

Scientists who previously supported older theories don’t need apologies, and as scientists they know — or should know — how it works. What you want (if I understand correctly) would turn factual problems into personal ones, which is not a good thing most of the time.

For me a relevant question would be, whether Carl Sagan even made a scientific statement when he talked about the Copernican Principle. It could very well have been some kind of publicity speech (as only one possibility out of several); then I would say, the whole case is overvalued.

Interstellar Bill December 25, 2011 at 5:46
” In this case it’s Carl Sagan’s beloved Copernican Principle, which has been decisively disproved by our own neat & tidy solar system being overwhelmingly untypical.”

The Copernican Principle was not invented my Carl Sagan , it is a basic , and wise, mode of thinking in science.
All good scientists follow it.
Every time humans have supposed something unique about themselves it has proved to be dead smooth wrong!

(Now that we know planetary systems a abundant, and about to become more abundant, a subset of them will be like the solar system, making us non unique.)

In this case it’s Carl Sagan’s beloved Copernican Principle, which has been decisively disproved by our own neat & tidy solar system being overwhelmingly untypical.

Who says our solar system is “untypical”? Every single planetary system observed so far has been unique. There is NO SUCH THING as a “typical” planetary system with which our own solar system’s configuration can be compared to as being like or unlike it.

Our solar system is as “untypical” as a poker hand consisting of a 3 of diamonds, a jack of clubs, a 8 of hearts, a 10 of diamonds, and a king of hearts.

Kepler’s findings have been completely and utterly consistent with the Copernican principle.

1) There are other planetary systems out there – ours is not the only one (consistent with the Copernican Principle)

2) There are systems with gas giants like Jupiter and Saturn out there – ours is not the only one (consistent with the Copernican Principle)

3) There are systems with smaller rocky planets out there – ours is not the only one (consistent with the Copernican Principle)

4) There are systems with mid-sized ice/gas planets out there like Neptune and Uranus – ours is not the only one (consistent with the Copernican Principle)

5) There are systems with belts of debris out there – ours is not the only one (consistent with the Copernican Principle)

6) Planetary systems form from collapsing protostellar discs, just as ours did (consistent with the Copernican Principle)

7) There are systems out there showing evidence of planetary migration after formation, just as ours did (consistent with the Copernican Principle)

8) There are planets out there of similar size to the Earth (consistent with the Copernican Principle)

9) There are planets out there of similar distance to their stars as Earth, receiving similar amounts of energy (consistent with the Copernican Principle)

Indeed, it would have been a violation of the Copernican Principle if we found that all other planetary systems out there were just like our own, because that would indicate that there is something special about how ours formed, such that all others must also form in the same manner, and that is a profoundly UNcopernican concept.

I think that Bill’s statement has a bit more pertinence than some would acknowledge here. To me the Copernican Principle may have no technical bearing on the science itself, but it has incredible influence on scientists, and thus the pathways of scientific discovery. So far the signal in the data from that path has also been that our surroundings are stunningly mediocre, but let us indulge the possibility that this will change.

Bill seems especially interested in using how Earth-like planets are as the next test, but to apply that to the CP we would have to be very careful. If Earth-type planets are rare this would not break the CP if planets were found to be common and a very high proportion of those that were equally narrowly categorised were found to be of equally rare type.

Here is another way to think of it Bill. If the Earth is the only planet that we know of that contains life we are justified assuming that the average life bearing planet would resemble Earth more than it does the average planet. However, without more data, we have no reason to believe that the average living planet resembles Earth more closely than Earth resembles the average of the most Earth-like planet taken from each multiplanet system.

Gosh! On rereading I realise that I explained my partial agreement with Bill poorly. A better statement is to say that once all measurement biases are well accounted for, it would be a valid test of how well the CP holds to look at how the Sol system deviates from the norm from a long list of pre-chosen and maximally independent parameters. On applying Chi squared to this set we would not want the Sol system to be in the top 1% of deviant systems, nor for that matter be in the bottom 1% of that deviancy.

Note that somehow Bill and allies must have selected the parameters used in a way that is completely unbiased by observations that have already come in, and note that the three sigma signal that I have thought to be at the threshold of interest is considered weak in particle physics. Finally note that “we are not specially privileged” has to continue being the default assumption for analytic reasons, no matter what we think of the CP.

How well do major American and British newspapers report space science news? The answer matters because it is print rather than broadcast news that continues to do the heavy lifting in communicating complex information. Even when the general audience is reading online about events involving space the source is typically a newspaper. Comparing the coverage in major American and British newspapers of the discovery of extrasolar planets Kepler 20e and 20f on December 21 offers interesting insights.

What distinguished good from bad reporting of this story was that it both identified what was surprising about this discovery and provided context with background about detecting extrasolar planets and comparable previous discoveries. So rather than leave this particular news story suspended in isolation, it is integrated in a stream of news so that readers may construct a deeper understanding of the subject.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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